328 MEMOIRS OF THE CARNEGIE MUSEUM 
All modifications of the gills are directly or indirectly correlated with the 
sexual apparatus, and besides a number of other characters of the soft parts as well 
as the hard parts are connected with it; in fact the sexual apparatus is the most 
important feature of the Najades. This is not at all astonishing; we find the same 
conditions in so many other groups of animals. Thus it is clear that a system 
founded upon these characters must be a natural one, and that it shou'd be fitted 
to furnish us the key for the proper understanding of the systematic relations. 
Thus we are to regard the above three groups (excluding Margaritana) as so 
many natural divisions of the family Unronipm, which then, of course, should 
rank as subfamilies,"”) namely: the UNIONINa&/, ANODONTIN#!, and LAMPSILIN®. For 
Margaritana it is unquestionably best to create a new family, the MARGARITANIDA. 
It is evident, even without the knowledge of certain details of the sexual appa- 
ratus, that the Margaritanide form the most primitive group of the Najades. The 
character of the gills surely indicates this, and also the character of the branchial and 
anal openings, although possibly the shell may not represent the most ancient type. 
Of the Unionide, the Unionine are certainly more primitive than the other 
two subfamilies, as is evidenced by the simple character of the structure of the 
marsupial gills. The Anodontine and Lampsiline are more advanced, but they 
have advanced in different directions, and each has developed special features 
of the sexual apparatus. Generally speaking, the Lampsiline contain the most 
highly advanced types, as is shown by the restriction of the marsupium to a part 
of the outer gill, and by the strong expression of the sexual differentiation in the 
outer shell. Yet there are forms among the Anodontine, which show extremely 
complex structures (Strophitus), unparalleled in any other genus, and the peculiar 
glochidia of the Anodontine surely mark a high stage of development. The ar- 
rangement and sequence of the families and subfamilies would possibly be the 
best as given above; but we must not forget that, while, for instance, the Lamp- 
siline show relations in the structure of the edge of the gill to the Anodontine, 
they more nearly approach the Unionine in the shape of the glochidia. This is, 
however, the usual difficulty in all our systematic arrangements. 
“Simpson unites all these forms in one subfamily, Unionine, from which he distinguishes as a second subfamily the 
Hyriane (properly Hyriine), with the marsupium formed by the inner gills alone, and with radial beak sculpture. The 
conception of the latter subfamily on the part of Simpson is entirely wrong. Very little is known about their anatomy, but 
I recently have been able to investigate some of the genera, and find that some are really Unionine (in the new sense), while 
others rather agree with the family Mutelide (See Nautilus, 23, April, 1910, p. 139, and January, 1911). With regard to 
the sculpture of the beak, serious mistakes have been made. The shape of the marsupium certainly is important, but at pres- 
ent in many genera is unknown. The Mutelide are reported to possess a lasidium in place of the glochidium, but again the 
larvee of many genera of these are entirely unknown. I hope to be able to contribute to the solution of this question in 
the near future, on the basis of a splendid collection representing the ‘Hyriine’’ from South America. 
